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SUPA initiatives
Nuclear and Plasma Physics
Nuclear astrophysics
The light emitted by the stars is a direct result of the nuclear reactions
that naturally occur when massive clouds of gas collapse and heat, whether
it be the constant shine of our Sun shown above, or the newly observed flares
of gamma-ray bursters. The abundances of the elements here on Earth are the
result of nuclear reactions that have occurred in previous generations of
stars. The understanding of nucleosynthesis processes and of the energy generation
in astrophysical objects is the subject of Nuclear Astrophysics studied experimentally
by the Edinburgh Nuclear
Physics Group.
Explosive Hydrogen Burning
Accretion from a hydrogen rich envelope of one star onto the electron degenerate
surface of a compact evolved companion can generate the conditions required
for a thermonuclear runaway. In a binary system such a case might be when
material from the outer layers of a red giant is transferred on to the surface
of a white dwarf. Such an event is what we observe as a classical nova, and
understanding the sequence of nuclear reactions that occur is the key to
understanding the observed light curves and elemental abundances in the ejecta.
Similarly, X-ray bursters can be interpreted as accretion on to a neutron
star. In this case the deeper gravitational potential wells lead to hotter
temperatures and higher pressures, and consequently the nuclear reaction
pathways follow faster, more extreme routes. The rate of key nuclear reactions
in explosive nucleosynthesis can be directly determined using radioactive
beams.
Measuring Explosive Reactions here on Earth
Explosive reactions involve radioactive nuclear species. A major advance
in the last decade has been the development of accelerators capable of producing
intense beams of radioactive isotopes. This is the only direct way to measure
these key reactions that control explosive astrophysical events. To the left
is shown the pioneering two cyclotron accelerator
complex at Louvain la Neuve, Belgium.
Hot Cycling in Stars
To
the right is shown one of the hot CNO cycles that ignites on the surface
of white dwarfs in novae explosions. The rate of energy generation in such
processes critically depends on reactions involving radioactive nuclear species.